Excitons in Two-Dimensional Semiconductors—a Momentum-Resolved Perspective

Optical techniques have provided us with rich information about the exciton – a two-particle photoexcited state in semiconductors and insulators. Yet, they have left a fundamental degree of freedom of the exciton inaccessible – it’s <i>momentum</i>!<br/>Over the past decade, we have developed novel time-resolved photoemission spectroscopy techniques that have yielded valuable insights into perovskite photovoltaic materials and other semiconductor structures [1, 2, 3, 4]. In this talk, I will discuss the application of these techniques to access the momentum coordinate of excitons in 2D semiconductors, thereby providing us the formation pathways of momentum-forbidden dark excitons [5], an image of the distribution of the electron around the hole in the exciton [6], and the degree of localization of the center of mass coordinate of the exciton in a moiré cell [7].<br/>References<br/>[1] M. K. L. Man, et al. <i>Nature Nanotechnol.</i> <b>12</b>, 36 (2017)<br/>[2] E. L. Wong, et al. <i>Science Advances</i> <b>4</b>, eaat9722 (2018)<br/>[3] T. Doherty^*, A. Winchester^*, et al. <i>Nature</i> <b>580</b>, 360 (2020)<br/>[4] S. Kosar, et al, <i>Energy and Environment Sciences</i>, DOI: 10.1039/d1ee02055b (2021)<br/>[5] J. Madeo^*, M. K. L. Man^*, et al. <i>Science</i> <b>370</b>, 1199 (2020).<br/>[6] M. K. L. Man^*, J. Madeo^*, et al. <i>Science Advances</i> <b>7</b>, eabg0192 (2021).<br/>[7] O Karni^*, E. Barr^*, V. Pareek^*, J. D. Georgaras^*, M. K. L. Man^*, C. Sahoo^*, et al. <i>arXiv</i>:2108.01933 (2021)<br/>^* equal authors